1) Field of the Invention
This invention relates to a double-sided printing apparatus suitable for use for electrophotographic printing on front and rear surfaces of continuous recording paper by a plurality of image forming stations and fixing stations disposed in a single apparatus.
2) Description of the Related Art
Conventionally, several techniques are used to print on front and rear surfaces of a recording medium (hereinafter referred to as medium) such as continuous recording paper by means of a printing apparatus of the electrophotographic type. For example, one of generally known techniques employs two single-sided printing apparatus (hereinafter referred to individually as first single-sided printing apparatus and second single-sided printing apparatus for convenience of description) each of which can print only on one surface of a medium and which are arranged in series along a transportation direction of a medium, and a reversing apparatus interposed between the two single-sided printing apparatus for reversing a medium between the front and rear surfaces.
According to the technique described above, one of the front and rear surfaces of a medium is first printed by the first single-sided printing apparatus, and then the medium is reversed by the reversing apparatus, whereafter the thus reversed medium is supplied to the second single-sided printing apparatus so that the other surface of the medium is printed by the second single-sided printing apparatus, thereby printing both of the front and rear surfaces of the medium.
Also a technique wherein double-sided printing of a medium is performed by a single printing apparatus is known. According to the technique, a medium is transported in a horizontal direction in the single apparatus, and an image forming process section for forming a toner image on an upper surface of the medium and a fixing station for fixing the toner image formed on the upper surface of the medium are disposed above the medium in the apparatus while another image forming process section for forming a toner image on a lower surface of the medium and another fixing station for fixing the toner image formed on the lower surface of the medium are disposed below the medium in the single apparatus such that printing on the two surfaces of the medium is performed while the medium is transported in the printing apparatus.
However, the conventional double-sided printing apparatus described above have the following subjects to be solved.
In particular, the double-sided printing apparatus which employs two single-sided printing apparatus has a subject to be solved in that, since it is necessary to dispose the two single-sided printing apparatus in a juxtaposed relationship with each other and dispose a reversing apparatus for reversing a medium between the two single-sided printing apparatus, the apparatus is large in size and particularly requires a large installation area.
On the other hand, in the double-sided printing apparatus wherein a medium is transported horizontally in the single apparatus and image forming process sections and fixing stations are arranged above and below the medium, since the image forming process sections are located above and below the medium, the image forming process section at the upper position and the image forming process section at the lower position exhibit different directions in which they contact with the medium, and consequently, the two image forming processing sections cannot be formed from common parts.
In particular, since conditions for formation of an image are different between the image forming process section which is disposed above the medium and forms a toner image on the upper surface of the medium and the image forming process section which is disposed below the medium and forms a toner image on the lower surface of the medium, setting conditions and arrangement conditions of parts of a developer, a precharger, an exposure member and so forth with respect to a photosensitive drum are different between the image forming process sections. Consequently, parts which compose the image forming process section disposed above the medium and parts which compose the image forming process section disposed below the medium have different constructions from each other.
Accordingly, since it is necessary to develop and produce two kinds of image forming process sections including the image forming process section to be disposed above the medium and the image forming process section to be disposed below the medium, there is a subject to be solved in that the cost and the time are required as much for development and so forth of them and a high production cost is required as much.
Further, since also consumables such as a developer, a developing unit itself and a photosensitive drum used in the two kinds of image forming process sections are different in construction from each other, two kinds of products must be prepared for each consumable. Consequently, also the expense and the time are required as much for development and so forth of the consumables and a high production cost is required as much. Further, there is a subject to be solved that, upon replacement of the consumables, the operator must pay attention so as not to mistake which one of two kinds of consumables should be used, and time is required as much.
Also a further double-sided printing apparatus is known which solves the subjects described above by forming two image forming process sections in a common construction. To this end, according to the double-sided printing apparatus, a medium is transported in a substantially vertical direction (such transportation is hereinafter referred to as vertical transportation) in the single printing apparatus and image forming process sections and fixing sections are disposed adjacent the opposite surfaces of the medium so that the imaging forming process sections and the fixing sections may be individually composed of common parts.
However, continuous paper which is used as a medium in a printing apparatus is used for high speed printing (for example, approximately 8,000 lines/minute for one surface) from its advantages that it is less likely to suffer from paper jamming upon transportation thereof, that it does not require such an operation as picking, and so forth. And, in order to allow such high speed printing in a printing apparatus, the diameters of a photosensitive drum and a developing roller of an image forming process section must be large. Thus, the conventional double-sided printing apparatus wherein a medium is transported vertically in the single apparatus has a subject to be solved in that, if the apparatus is constructed merely such that a medium is transported vertically and image forming process sections and fixing sections are successively disposed in the vertical direction on the opposite sides of the medium, then it has a great vertical dimension or height.
Where the height of the apparatus is great, it follows that some part of the medium is transported at a high position. This makes it difficult to perform an operation for a medium such as, for example, an operation of removing jamming paper (medium) when paper jamming or the like occurs. Further, since also a printing unit such as an image forming process section or a fixing station is disposed at a high position, such an operation as maintenance or checking cannot be performed readily, resulting in a subject that the workability is low. Therefore, where the workability is taken into consideration, the height of the apparatus is preferably set so that the operator can operate the apparatus readily by hand (for example, approximately 1,500 mm).
Meanwhile, in printing by electrophotography, as a fixing unit for fixing a toner image formed on a medium by each image forming process section, a fixing unit including heat rollers which contact with and are driven to rotate by a medium being transported, a flash fixing unit for fixing a toner image by means of a flash lamp such as a xenon lamp or some other fixing unit is used.
In a fixing unit which includes heat rollers, when a medium is held by and between the heat rollers and is transported in order to perform fixing, the temperature of the heat rollers drops. Further, if the medium is transported at a high speed in order to assure a high printing speed, then the temperature of the heat rollers drops remarkably. This makes it difficult to maintain a desired temperature for fixing of a toner image and hence to maintain the printing quality. Further, since the heat rollers of a high temperature are pressed against the medium upon fixing, there is the possibility that the medium may be damaged.
On the other hand, a flash fixing unit exhibits a less influence upon a medium than a fixing unit which employs heat rollers. However, since flash light of the flash fixing unit is very intense, there is a subject to be solved in that light leaking from between a gap between the flash fixing unit and the medium or the like is irradiated upon photosensitive drums of image forming process sections and the photosensitive drums are optically deteriorated by the leaking light, resulting in reduction of the life of the photosensitive drums. Further, local optical deterioration of the photosensitive drums by the leaking light causes an irregular printing density, resulting in deterioration of the printing quality. Further, the leaking light drops the surface potentials of the photosensitive drums. Also this gives rise to a subject to be solved in that the printing quality is deteriorated.
Particularly around a portion of a transport path of a medium in the apparatus where the medium does not pass, leaking light from a flash fixing unit is not interrupted by the medium or some other element, and this intense leaking light is directly irradiated upon the photosensitive drums. Therefore, deterioration of the photosensitive drums, a drop of the surface potentials and so forth are likely to occur remarkably.
Further, in a double-sided printing apparatus in which flash fixing is involved, toner powder transferred to printing surfaces of a medium is heated upon emission of flash light by fixing units, and smoke, odor and so forth composed of high molecular organic substances such as styrene, butadiene and phenol are produced from around the fixing units. Therefore, in a double-sided printing apparatus which employs flash fixing, in order to remove such smoke and so forth, gas discharging processing apparatus including ducts, fans and activated carbon filters are provided individually for a fixing unit for a recording medium front surface and a fixing unit for a recording medium rear surface so that smoke and so forth generated may be attracted and discharged by the gas discharging processing apparatus.
However, in a double-sided printing apparatus which employs flash fixing, in order to detect timings at which the activated carbon filters should be replaced, pressure sensors or the like are provided for the individual filters, and choking of the activated carbon filters is detected from detection values of the pressure sensors to discriminate the timing for replacement. However, since the frequency of use is different between the fixing unit for the front surface and the fixing unit for the rear surface of the medium, it is necessary to provide pressure sensors or the like for both of the filter attached to the fixing unit for the front surface and the filter attached to the fixing unit for the rear surface of the medium and supervise the pressure sensors separately from each other. Consequently, there is a subject to be solved in that a high production cost is required for the apparatus as much.